Organic light-emitting device with improved layer conductivity distribution

ABSTRACT

An OLED comprises an anode, a hole source, an emissive region, an electron source and a cathode, wherein the materials for the electron source and the hole source are chosen such that the electrical conductivity of these charge carrier sources is greater than the electrical conductivity of the emissive region. In particular, the electrical conductivity of the source layers is between 10&lt;SUP&gt;-8 &lt;/SUP&gt;to 10&lt;SUP&gt;2 &lt;/SUP&gt;S/cm. Furthermore, one or both of the hole source and the electron source are made substantially of one or more inorganic materials. The emissive region can have one or more layers of organic material. The materials for the emissive region are insulators. The cathode can be made of a metal such as Mg:Ag and Al, and the anode is made of ITO or the like. The electrical conductivity of the cathode and the anode is significantly higher than 10&lt;SUP&gt;2 &lt;/SUP&gt;S/cm.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention is related to a co-pending application Ser. No.10/995,878, filed Nov. 23, 2004, assigned to the assignee of the presentinvention.

FIELD OF THE INVENTION

The present invention relates generally to photoelectric devices and,more specifically, to organic light-emitting diodes.

BACKGROUND OF THE INVENTION

Organic light-emitting diodes (OLEDs) are known in the art. For example,Hung et al. (U.S. Pat. No. 5,776,623) also discloses anelectroluminescent device wherein a 15 nm-thick CuPc layer is used as ahole injection layer (HIL), a 60 nm-thick NPB layer is used as a holetransport layer (HTL), a 75 nm-thick Alq₃ layer is used as an electrontransport layer (ETL). A 0.5 nm-thick lithium fluoride layer is alsodeposited on the Alq₃ layer. The lithium fluoride layer can be replacedby a magnesium fluoride, a calcium fluoride, a lithium oxide or amagnesium oxide layer.

Kido et al. (U.S. Pat. No. 6,013,384) discloses, as shown in FIG. 1 a,an organic electroluminescent device 10 wherein the optoelectronicsub-structure consists of a hole transport layer (HTL) 13, a luminescentlayer 14 and a metal-doped organic compound layer 15 disposed between ananode layer 12 and a cathode layer 16. The device is fabricated on asubstrate 11.

Weaver et al. (U.S. Publication No. 2004/0032206 A1) discloses anotherOLED including an alkali metal compound layer. As shown in FIG. 1 b, theOLED 20 is fabricated on a plastic substrate 21 pre-coated with an ITOanode 22. The cathode consists of two layers: a metal oxide layer 28deposited over a layer 27 of Mg or Mg alloy, such as an alloy of Mg andAg. The alkali metal compound layer 26 can be made of alkali halides oralkali oxides such as LiF and Li₂O. The organic layers include an HTLlayer 23, an emissive layer (EML) 24 and an electron transport layer(ETL) 25.

Raychaudhuri et al. (U.S. Pat. No. 6,551,725 B2) discloses an OLED 30wherein a buffer structure is disposed between the organic layer and thecathode. As shown in FIG. 1 c, the buffer structure consists of twolayers, a first layer 37 containing an alkali halide is provided overthe electron transport layer (ETL) 36, and a second buffer layer 38containing a metal or metal alloy having a work function between 2.0 and4.0 eV is provided over the first buffer layer 37. In addition, a holeinjection layer (HIL) 33 is provided between the anode 32 and theorganic layers. The hole injection layer can be made of a porphorinic orphthalocyanine compound. The hole injection layer can also be made of afluorinated polymer CF_(x), where x is 1 or 2. The hole transport layer(HTL) 34 can be made of various classes of aromatic amines. The emissivelayer (EML) 35 provides the function of light emission produced as aresult of recombination of holes and electrons in the layer. The cathodelayer 39 is made by sputter deposition to provide increased conductivityand reflectivity of the electron injection layer of the device.

A generalized OLED structure is shown in FIG. 2. The hole injection andtransport layers together can be treated as a hole source. The electroninjection and transport layers together can be treated as an electronicsource. One or both the electron source and the hole source can be madeof organic or inorganic materials. The emissive layer is made of anorganic host material doped with a fluorescent or phosphorescent dopant.In general, the electrical conductivity of prior art interlayers islower than 10⁻⁸ S/cm (S=Ω⁻¹), limiting the light-emitting efficiency ofthe device.

It is known that a PIN diode is a photoelectric device with a large,neutrally doped intrinsic region sandwiched between p-doped and n-dopedsemiconducting regions. The doping in the p-doped and n-doped regionssignificantly increases the electrical conductivity of the semiconductormaterial and the efficiency of the device.

It would be desirable and advantageous to improve the device efficiencyof an OLED by changing the electrical conductivity distribution invarious layers in the OLED.

SUMMARY OF THE INVENTION

It is an object of the present invention to improve the deviceefficiency of an organic light-emitting device (OLED). This object canbe achieved by choosing the materials for the electron source and thehole source such that the electrical conductivity of these source layersis greater than the electrical conductivity of the emissive region. Inparticular, the electrical conductivity of the source layers is between10⁻⁸ to 10² S/cm. Furthermore, one or both of the hole source and theelectron source are made substantially of one or more inorganicmaterials. The emissive region can have one or more layers of organicmaterial. The materials for the emissive region are insulators.

In the OLED of the present invention, the cathode can be made of a metalsuch as Mg:Ag and Al, and the anode is made of ITO or the like. Thus,the electrical conductivity of the cathode and the anode issignificantly higher than 10² S/cm. A thin layer of LiF can be disposedbetween the cathode and the electron source to serve as an electroninjection layer.

Furthermore, one or more of the hole and electron source layers are madeof an inorganic material. One of the hole and electron source layers canbe made of an ion-doped organic material.

The present invention will become apparent upon reading the descriptiontaken in conjunction with FIGS. 3 to 12.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic representation showing a prior art organiclight-emitting diode.

FIG. 1 b is a schematic representation showing another prior art organiclight-emitting diode.

FIG. 1 c is a schematic representation showing yet another prior artorganic light-emitting diode.

FIG. 2 is a schematic representation showing a generalized OLED.

FIG. 3 is a schematic representation showing the electrical conductivitydistribution among various regions of the OLED, according to the presentinvention.

FIG. 4 a is a schematic representation showing the electricalconductivity distribution in an exemplary OLED, according to the presentinvention, wherein the hole source is made of an organic material andthe electron source is made of an inorganic material.

FIG. 4 b is a schematic representation showing the electricalconductivity distribution in an exemplary OLED, according to the presentinvention, wherein the hole source is made of an organic material andthe electron source comprises an inorganic and an organic layer.

FIG. 4 c is a schematic representation showing the electricalconductivity distribution in another exemplary OLED, according to thepresent invention, wherein the hole source is made of an organicmaterial and the electron source also comprises an inorganic layer andan organic layer.

FIG. 5 a is a schematic representation showing the electricalconductivity distribution in an exemplary OLED, according to the presentinvention, wherein the electron source is made of an organic materialand the hole source is made of an inorganic material.

FIG. 5 b is a schematic representation showing the electricalconductivity distribution in an exemplary OLED, according to the presentinvention, wherein the electron source is made of an organic materialand the hole source comprises an inorganic layer and an organic layer.

FIG. 5 c is a schematic representation showing the electricalconductivity distribution in another exemplary OLED, according to thepresent invention, wherein the electron source is made of an organicmaterial and the hole source also comprises an inorganic layer and anorganic layer.

FIG. 6 is a schematic representation showing the electrical conductivitydistribution in an exemplary OLED, according to the present invention,wherein both the electron source and the hole source are made ofinorganic materials.

FIG. 7 is a schematic representation showing the electrical conductivitydistribution in an exemplary OLED, according to the present invention,wherein each of the electron and hole sources comprises an inorganiclayer and an organic layer.

FIG. 8 a is a schematic representation showing the materials in anexemplary OLED, according to the present invention.

FIG. 8 b is a schematic representation showing the materials in anotherexemplary OLED, according to the present invention.

FIG. 8 c is a schematic representation showing the materials in yetanother exemplary OLED, according to the present invention.

FIG. 9 a is a schematic representation showing an OLED, according to anembodiment of the present invention, wherein the hole source is made ofan organic material.

FIG. 9 b is a schematic representation showing an OLED, according toanother embodiment of the present invention, wherein the hole source ismade of an organic material.

FIG. 9 c is a schematic representation showing an OLED, according to yetanother embodiment of the present invention, wherein the hole source ismade of an organic material.

FIG. 10 a is a schematic representation showing an OLED, according to anembodiment of the present invention, wherein the electron source is madeof an organic material.

FIG. 10 b is a schematic representation showing an OLED, according toanother embodiment of the present invention, wherein the electron sourceis made of an organic material.

FIG. 10 c is a schematic representation showing an OLED, according toyet another embodiment of the present invention, wherein the electronsource is made of an organic material.

FIG. 11 is a schematic representation showing an OLED, according to anembodiment of the present invention, wherein both the electron and holesources are made of inorganic materials.

FIG. 12 is a schematic representation showing an OLED, according to anembodiment of the present invention, wherein each of the electron andhole sources comprises an inorganic material and an organic material.

DETAILED DESCRIPTION OF THE INVENTION

The organic light-emitting device (OLED), according to the presentinvention, comprises a cathode, an electron source, an emissive layer, ahole source and an anode. The materials for use in the electron sourceand the hole source are chosen such that the electrical conductivity ofthe electron source and the hole source is higher than that of theemissive layer, but significantly lower than that of the cathode and theanode. In particular, the electrical conductivity of the emissive layeris lower than 10⁻⁸ S/cm, and the electrical conductivity of the sourcelayers is between 10⁻⁸ to 10² S/cm, as illustrated in FIG. 3. If thecathode is made of a metal such as Ag and Al, then its electricalconductivity is higher than 10⁶ S/cm. If the anode is made of ITO, thenits electrical conductivity is between 10³ to 5×10³ S/cm.

One or both of the electron source and the hole source in the OLED,according to present invention, are made of an inorganic material havingan electrical conductivity greater than 10⁻⁸ S/cm. FIGS. 4 a-4 c areschematic representations showing the electrical conductivitydistribution in an exemplary OLED, according to the present invention,wherein the hole source is made of an organic material. The OLED has acathode structure, which can be a single electrode or a combination ofan electrode and a thin layer of electron injection material. Theelectron source can be entirely made of an inorganic material. Theelectron source can be one layer of the same material or a plurality oflayers of different materials. The electron source can be made of aninorganic material, as shown in FIG. 4 a. However, the electron sourcecan have an inorganic layer and an organic layer. The inorganic layercan be located adjacent to the cathode structure, as shown in FIG. 4 b,or the inorganic layer can be located adjacent to the emissive region,as shown in FIG. 4 c.

FIGS. 5 a-5 c are schematic representations showing the electricalconductivity distribution in an exemplary OLED, according to the presentinvention, wherein the electron source is made of an organic material.The hole source can be made of an inorganic material, as shown in FIG. 5a. However, the hole source can have an inorganic layer and an organiclayer. The organic layer can be located adjacent to the anode, as shownin FIG. 5 b, or the organic layer can be located adjacent to theemissive region, as shown in FIG. 5 c.

FIG. 6 is a schematic representation showing the electrical conductivitydistribution in an exemplary OLED, according to the present invention,wherein both the electron source and the hole source are made ofinorganic materials.

FIG. 7 is a schematic representation showing the electrical conductivitydistribution in an exemplary OLED, according to the present invention,wherein each of the electron and hole sources comprises an inorganiclayer and an organic layer.

In order to increase the electrical conductivity of the hole source andthe electron source, it is desirable to use a semi-metal for thesesources. A semi-metal has an electrical conductivity between 10⁻⁸ to 10²S/cm. For example, ion-intercalated inorganic compound and ion-dopedorganic material can be used to achieve the electrical conductivity of10⁻⁶ S/cm and higher. Further exemplary embodiments of the OLED,according to the present invention, are shown in FIGS. 8 a to 8 c.

FIGS. 8 a-8 c is are schematic representations showing the materials inthe OLED, according to the present invention. As shown in FIG. 8 a, theelectron source contains an ion-intercalated inorganic material which isan N-type semi-metal, and the hole source contains an ion-doped organicmaterial which is a P-type semi-metal. The emission region contains oneor more organic materials. As shown in FIG. 8 b, the hole sourcecontains an ion-intercalated inorganic material which is a P-typesemi-metal, and the electron source contains an ion-doped organicmaterial which is an N-type semi-metal. The emission region contains oneor more organic materials. As shown in FIG. 8 c, the electron sourcecontains an ion-intercalated inorganic material which is an N-typesemi-metal, and the hole source contains an ion-intercalated organicmaterial which is a P-type semi-metal. The emission region contains oneor more organic materials.

The ion-intercalated inorganic compound for use in the electron sourcecan be an oxide-based alkali or alkaline-earth metal compound, forexample. The oxide-based inorganic compound can be selected from a groupof metal-oxides characterized by the chemical formula ofA_(x)(M_(y)O_(z)), where

-   -   x, y, z are positive integers greater than zero;    -   A is selected from the group consisting of alkali and        alkaline-earth elements;    -   M is selected from the group consisting of metals, transitional        metals and alloys; and    -   O is an oxygen atom.

Some of the ion-intercalated inorganic compounds are LiMn₂O₄, LiCoO₂,LiNbO₃, Li₂WO₄, Cs₂WO₄, CsMnO₄, CsVO₄, CsTi₆O₁₃, MgTiO₃, MgWO₄, MgZrO₃,Li(Ni_(0.8)Co_(0.2))O₂. In the above compounds:

-   -   Li=Lithium    -   Mn=Manganese    -   O=Oxygen    -   Co=Cobalt    -   Nb=Niobium    -   W=Tungsten    -   Cs=Cesium    -   V=Valadium    -   Ti=Titanium    -   Mg=Magnesium    -   Zr=Zirconium    -   Ni=Nickel

The ion-intercalated inorganic compound for use in the hole source canbe an inorganic compound selected from a group of oxides characterizedby having the chemical formula of P_(x)(M_(y)O_(z)), where

-   -   x, y, z are positive integers greater than zero;    -   P is a p-dopant, such as tetrafluoro-tetracyano-quinodimethane        (F₄-TCNQ);    -   M is selected from the group consisting of metals, transitional        metals and alloys; and    -   O is an oxygen atom.

An example of the ion-doped organic material is p-dope amine, which hasan electrical conductivity in the range of 4×10⁻⁷ to 6×10⁻⁶ S/cm.Organic material not doped with ions usually has an electricalconductivity lower than 10⁻⁹ S/cm and can be considered as an electricalinsulator. Another example of the ion-doped organic material is Li-dopedorganic material which has an electrical conductivity in the range of2×10⁻⁵ to 5×10⁻⁵ S/cm.

Exemplary Embodiments

FIGS. 9 a-12 show different embodiments of the present invention. FIGS.9 a-9 c are exemplary embodiments of the present invention, wherein thehole source is made of an organic material. As shown in FIG. 9 a, thehole source contains a hole injection layer (HIL) of F₄-TCNQ dopedcopper-phthalocyanine (CuPc). CuPc: F₄-TCNQ is a p-type dopant. Theelectron source comprises an electron transport layer (ETL) of LiMn₂O₄,an ion-intercalated inorganic material. The emissive region containsthree layers: an emissive layer (EML), an N-buffer layer disposedbetween the emissive layer and the electron source and a P-buffer layerdisposed between the emissive layer and the hole source. For example,the emissive layer contains an organic host doped with a red-emittingluminescent dopant; the N-buffer layer is made of4,7-diphenyl-1,10-phenan-throline (BPhen); and the P-buffer layer ismade of N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′ (NPB).The N-buffer layer is effectively another electron transport layer andthe P-buffer layer is effectively a hole transport layer (HTL). Thecathode structure comprises an aluminum electrode and a thin layer ofLiF as an electron injection layer (EIL).

In addition to the LiMn₂O₄ layer, the electron source contains anotherelectron transport layer made of BPhen:Li. In the embodiment as shown inFIG. 9 b, the BPhen:Li layer is disposed between the LiMn₂O₄ layer andthe emissive region. In the embodiment as shown in FIG. 9 c, theBPhen:Li layer is disposed between the LiMn₂O₄ layer and the cathode. Inthe embodiments as shown in FIGS. 9 b and 9 c, the cathode structurecomprises only an electrode made of Mg:Ag.

FIGS. 10 a-10 c are exemplary embodiments of the present invention,wherein the electron source is made of an organic material. As shown inFIG. 10 a, the electron source contains an electron transport layer(ETL) of BPhen:Cs. The emissive region contains three layers: anemissive layer (EML), an N-buffer layer disposed between the emissivelayer and the electron source and a P-buffer layer disposed between theemissive layer and the hole source. For example, the emissive layercontains an organic host doped with a red-emitting luminescent dopant;the N-buffer layer is made of 4,7-diphenyl-1,10-phenan-throline (BPhen);and the P-buffer layer is made ofN,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′ (NPB). TheN-buffer layer is effectively another electron transport layer and theP-buffer layer is effectively a hole transport layer (HTL). In theembodiment as shown in FIG. 10 a, the hole source comprises a holeinjection layer of F₄-TCNQ doped tungsten oxide (WO₃) and the cathodestructure comprises an aluminum electrode and an electron injectionlayer of LiF.

In the embodiments as shown in FIGS. 10 b and 10 c, the cathodestructure contains a layer of Mg:Ag and the hole source furthercomprises another hole injection layer of NPB:F₄-TCNQ. In the embodimentas shown in FIG. 10 b, the NPB:F₄-TCNQ layer is disposed adjacent to theemissive region. In the embodiment as shown in FIG. 10 c, theNPB:F₄-TCNQ layer is disposed adjacent to the ITO anode.

FIG. 11 is a schematic representation showing an OLED, according to anembodiment of the present invention, wherein both the electron and holesources are made of inorganic materials. In FIG. 1, the electron sourcecomprises an electron transport layer of LiMn₂O₄ and the hole sourcecomprises a hole injection layer of F₄-TCNQ:WO₃. The cathode structurecomprises an aluminum electrode and an electron injection layer of LiF.

FIG. 12 is a schematic representation showing an OLED, according to anembodiment of the present invention, wherein each of the electron andhole sources comprises an inorganic material and an organic material. Asshown in FIG. 12, the electron source further comprises an electrontransport layer of BPhen:Li, and the hole source further comprises ahole injection layer of NPB:F₄-TCNQ.

In sum, in an OLED comprising a cathode structure, a hole source, anemissive region, an electron source and an anode, the present inventionmakes use of the electrical conductivity distribution among the holesource, the electron source and the emissive layer to improve the deviceefficiency. The materials for the hole source, the emissive region andthe electron source are such that the electrical conductivity in thehole source and the electron source is higher than that of the emissiveregion, but lower than that of the cathode and anode. In particular, theelectrical conductivity of the source layers is between 10⁻⁸ to 10²S/cm. Furthermore, one or both of the hole source and the electronsource are made substantially of one or more inorganic materials.

Furthermore, one or more buffer layers can be disposed between theelectron source and the cathode. These buffer layers are parts of theemissive region.

Thus, although the present invention has been described with respect toone or more embodiments thereof, it will be understood by those skilledin the art that the foregoing and various other changes, omissions anddeviations in the form and detail thereof may be made without departingfrom the scope of this invention.

1. An organic light-emitting device comprising: a cathode; an anode; anda layer structure disposed between the cathode and the anode; the layerstructure comprising: a hole source region disposed adjacent to theanode; an electron source region disposed adjacent to the cathode; andan emissive region disposed between the hole source region and theelectron source region, the emissive region made of at least one organichost material doped with one or more luminescent dopants, each of thehole source region, the electron source region and the emissive regionhaving an electrical conductivity, wherein the electrical conductivityof the hole source region is between 10⁻⁸ to 10² S/cm; the electricalconductivity of the electron source region is between 10⁻⁸ to 10² S/cm;the electrical conductivity of the emissive region is lower than 10⁻⁸S/cm; and the electrical conductivity of the cathode and the anode ishigher than 10² S/cm, and wherein at least one of the hole source andthe electron source regions comprises a layer made of an inorganicmaterial.
 2. The device of claim 1, wherein the hole source regioncontains a P-type semi-metal, the electron source region contains anN-type semi-metal, and the emissive region contains an electricalinsulator.
 3. The device of claim 1, wherein the hole source regioncomprises a layer of ion-intercalated inorganic material and theelectron source region comprises a layer of ion-doped organic material.4. The device of claim 1, wherein the hole source region comprises alayer of ion-intercalated inorganic material and the electron sourceregion comprises a layer of ion-intercalated inorganic material.
 5. Thedevice of claim 1, wherein the hole source region comprises a layer ofion-doped organic material and the electron source region comprises alayer of ion-intercalated inorganic material.
 6. The device of claim 1,wherein the emissive region comprises an emissive layer and a P-bufferlayer disposed between the emissive layer and the hole source region. 7.The device of claim 1, wherein the emissive region comprises an emissivelayer and an N-buffer layer disposed between the emissive layer and theelectron source region.
 8. The device of claim 1, wherein the electronsource region comprises one or more layers containing alkali or alkalineearth metal.
 9. The device of claim 1, wherein the electron sourceregion comprises a layer of LiMn₂O₄ and a layer of BPhen:Li.
 10. Thedevice of claim 1, wherein the hole source region comprises one or morelayers containing TCNQ.
 11. The device of claim 1, wherein the holesource region comprises a layer NPB:F₄-TCNQ and a layer of F₄-TCNQ:WO₃.12. The device of claim 1, further comprising an electron injectionlayer disposed between the cathode and the electron source region. 13.The device of claim 12, wherein the electron injection layer comprises alayer of LiF.
 14. A method for improving device efficiency of an organiclight-emitting device comprising: a cathode; an anode; and a layerstructure disposed between the cathode and the anode; the layerstructure comprising: a hole source region made of a first sourcematerial disposed adjacent to the anode; an electron source region madeof a second source material disposed adjacent to the cathode; and anemissive region disposed between the hole source region and the electronsource region, the emissive region made of at least one organic hostmaterial doped with one or more luminescent dopants, each of theemissive region, the hole source region and the electron source regionhaving an electrical conductivity, wherein at least one of the firstsource material and the second source material comprises an inorganicmaterial, said method comprising the steps of: introducing ions into thefirst source material so as to achieve the electrical conductivity ofthe hole source region between 10⁻⁸ to 10² S/cm; and introducing ionsinto the second source material so as to achieve the electricalconductivity of the electron source region between 10⁻⁸ to 10² S/cm;such that the electrical conductivity of the emissive region is lowerthan the electrical conductivity of the hole source region and theelectron source region; and that the electrical conductivity of thecathode and the anode is higher than the electrical conductivity of thehole source region and the electron source region.
 15. The method ofclaim 14, wherein the first source material comprises a layer ofion-intercalated inorganic material and the second source materialcomprises a layer of ion-doped organic material.
 16. The method of claim14, wherein the first source material comprises a layer ofion-intercalated inorganic material and the second source materialcomprises a layer of ion-intercalated inorganic material.
 17. The methodof claim 14, wherein the first source material comprises a layer ofion-doped organic material and the second source material comprises alayer of ion-intercalated inorganic material.
 18. The method of claim14, wherein the emissive region comprises an emissive layer and aP-buffer layer disposed between the emissive layer and the hole sourceregion.
 19. The method of claim 14, wherein the emissive regioncomprises an emissive layer and an N-buffer layer disposed between theemissive layer and the electron source region.
 20. The method of claim14, wherein the light-emitting device further comprises a layer of LiFdisposed between the cathode and the electron source region.